Studies On The Mechanism Of Olefin Polymerization Reactions Catalyzed By Metallocene Supported On Multimodal Mesoporous Silica

Compared with traditional Ziegler-Natta catalysts,the olefin polymerization mechanism of metallocene catalysts has been clearly and accurately understood.Of course,this is related to the structure of metallocene catalysts and the polymerization system.Conventional catalysts only have 1%to 10%of metals that can become active centers,while most metals in metallocene catalysts exist as single active centers.Therefore,it is difficult to obtain the information of active centers directly by analyzing the structure of traditional catalysts.Metallocene catalyzed olefin polymerization has very important practical significance.High activity,narrow molecular weight distribution and uniform copolymer distribution are its main characteristics.Compared with the development of foreign metallocene technology,domestic metallocene still has a large gap in catalyst preparation and performance.In this paper,metallocene catalysts,cocatalysts and silica gel were used as the main components to analyze the interaction mechanism between the three.The metallocene catalyzed olefin polymerization and their reaction mechanisms were elaborated,and the characteristics of the formed products were analyzed,including the following six chapters.The first chapter mainly introduces the development history and characteristics of metallocene catalysts;the mechanism of metallocene catalyzed olefin polymerization,the preparation and characteristics of supported silica for metallocene catalysts,and the development and mechanism of MAO for metallocene catalysts.The theory of density function is briefly introduced,with emphasis on Materials Studio software,including Dmol3,CASTEP and Adsorption Locator modules.Finally,the transition state theory is described.In the second chapter,the most stable cage structure?AlOMe?₄ was selected as the adsorbate,and the adsorption behavior of different crystals of cristobalite against MAO was calculated by Adsorption Locator.The adsorption behavior of MAO on different quartz crystal planes is quite different.Four adsorption points are formed on the SiO₂?100?crystal surface and the adsorption probability is the highest.Only one adsorption point is formed on the SiO₂?111?crystal surface,but the adsorption energy and chemical adsorption energy are the largest and the adsorption efficiency is the highest.The distribution of oxygen atoms in different crystal faces is related to the type of hydroxyl groups in amorphous silica gel.The SiO₂?111?plane corresponds to the free hydroxyl group,while the SiO₂?110?plane corresponds to the bridged hydroxyl group.Free hydroxyl groups play a crucial role in improving the adsorption efficiency of MAO.However,free hydroxyl groups are present in small pores inside the silica gel.In the third chapter,the repeated sol-gel process was used to prepare multi-peak mesoporous silica by adding alkaline medium and organic alcohol modifier.The spacing between the small pore diameter and the large pore diameter of silica gel can reach 20 nm or more.And the content of impurities contained is only 0.14 W%.It was found that at the initial temperature of 30°C,ammonia and organic alcohol were added at the initial stage of the sol,and organic alcohol was also added at the end of the gel.Silica has the best performance when the ratio between ammonia,organic alcohol and inorganic acid up to 2:3:4.Comparing the adsorption efficiency of different types of silica gel to MAO,the adsorption rate of multimodal mesoporous silica to MAO reaches95%,which is nearly 18.6%higher than that of conventional silica.This is critical for metallocene catalysts because the full use of MAO means a significant reduction in the cost of the metallocene catalyst.In the fourth chapter,the charge density,HOMO/LUMO and band gap of series metallocene compounds were calculated by DMOl3,and the two metallocene compounds?ⁿBuCp?₂ZrCl₂ and?1,3-Me,ⁿBuCp?₂ZrCl₂ were analyzed.The results show that the?ⁿBuCp?₂ZrCl₂ olefin polymerization process is carried out according to the?-agostic effect mechanism of the ground state and the transition state.During the polymerization process,a large amount of hydrogen is released,and the hydrogen content reaches 262 ppm,which causes the transition state barrier to appear before the low and then high.On the contrary,the?1,3-Me,ⁿBuCp?₂ZrCl₂ olefin polymerization process is carried out according to the Green-Rooney mechanism?hydrogen anion migration?,which produces small fragments such as propane,and the presence of unsaturated bonds and metal-H bonds in the polymer.This is also a prerequisite for the dehydrogenation reaction to occur.In the fifth chapter,the multi-peak pore distribution silica gel was used as the carrier,the compound?1,3-Me,ⁿBuCp?₂ZrCl₂ was the main catalyst,and the MAO was the auxiliary agent to form the metallocene polyethylene catalyst.The self-made catalyst is superior to the similar imported catalyst in the dispersion of morphology and auxiliary MAO,and the catalytic activity is released smoothly;however,the sensitivity to hydrogen is slightly worse than that of the imported catalyst.The ability of the two catalysts to catalyze the copolymerization of ethylene with an alpha-olefin is substantially equivalent.The performance comparison analysis of the three metallocene polyethylene products show that the 1-hexene content in mPE-1 is higher than that of mPE-2 in the case of similar density,indicating that the comonomer distribution in mPE-2 is not uniform.DSC analysis finds that mPE-2 form the smallest degree of branching,the highest crystallinity,and the thickness of the platelets.The TREF analysis finds that the mPE-1 fraction is mainly concentrated at 84?,while the mPE-2 also shows more fractions at 95?,confirming the presence of comonomer heterogeneity in the polymer.The two metallocene polyethylene films are blown and compared.The results show that the tensile and tear properties of mPE-1 are slightly worse than those of mPE-3,but the impact strength is better.In the sixth chapter,the preparation of metallocene polypropylene catalyst was completed by using multi-peak pore distribution silica gel as carrier.It was found by mode polymerization that the catalyst polymerization activity reached an optimum value when the amount of hydrogen added was 2.7 g and the amount of triisobutylaluminum was 20 mL.The metallocene polypropylene catalyst is very sensitive to hydrogen,as can be seen from the change in melt index.It is found that the metallocene polypropylene has a molecular weight distribution of only 2.5,an isotacticity of up to 99.4%,and an ash content of only 180 ppm through analysis,making it suitable for melt-blown or spunbonded nonwovens.The polypropylene solubles are only 0.9%,which reduces the deposits on the smoke and spinneret and makes the spinning process cleaner.The formed polymer has fine crystal grains,a crystallization temperature of only 152?,an elongation at break of 581.6%,and an impact strength of 1.8 KJ/cm²,thereby ensuring the production of fine denier filaments under stable spinning conditions.